Device to Device (D2D) is a well-known and widely used component of many existing wireless technologies when it comes to direct communication between wireless devices. D2D communications as an underlay to cellular networks have been proposed as a means to take advantage of the proximity of communicating devices and at the same time to allow devices to operate in a controlled interference environment. Typically, it is suggested that such D2D communication shares the same spectrum as the cellular system, for example by reserving some of the cellular uplink resources for D2D purposes. Allocating dedicated spectrum for D2D purposes is another alternative. For D2D communication to occur, the involved wireless device must have the same understanding of uplink subframe timing as the cellular network as they otherwise might overlap in time with cellular transmissions. Proximity Service (ProSe) should support D2D operations for wireless devices which are out of coverage from the network. In such a case where ProSe supports D2D communication for wireless devices which are out of coverage from the network, different synchronization options are possible: The out of coverage wireless devices may synchronize to a global reference (e.g., a GPS), which is in general different from the synchronization reference of deployed networks. Another possibility is that the out of coverage wireless device may operate in a fully asynchronous fashion (no synchronization reference, at least for discovery).
A wireless device which is ProSe enabled is required to work (discover and communicate with other ProSe enabled wireless devices) without network coverage.
However, the ProSe enabled wireless device is unable to achieve synchronization with the cellular system. A wireless device which is out of coverage from the cellular system will work in asynchronous mode, i.e. the wireless device transmits randomly in time and possibly with an offset in the frequency, which will cause severe interference to other wireless devices in its proximity and within the cellular network coverage, especially but not exclusively for a Time Division Duplex (TDD) system.
Furthermore, the wireless device has to use the resources that are preconfigured in the wireless device. The preconfigured resources are limited and lack flexibility (since they are pre-configured). This may lead to the high possibility of congestion when many wireless devices in proximity want to communicate simultaneously. Furthermore, wireless devices that are out of coverage have to use the power information preconfigured in the wireless device. The preconfigured power information cannot be adjusted according to the live/real network environment since they are out of coverage. In some cases, e.g. where there are several small cells deployed in dense, the preconfigured power might be too high to cause severe interference to the wireless devices within coverage. In other cases, e.g. where the wireless devices are scattered in a rather large area (e.g. 10 s meters), the preconfigured power might be too low to be heard by other wireless devices.